1. Field of the Invention.
The invention relates to a planar electromagnetic transducer that can be used in loudspeakers, headphones, microphones, or devices of a similar nature.
Planar magnetic loudspeakers have enjoyed distinct advantages over dynamic cone direct radiator type loudspeakers. They exhibit wider bandwidth and phase characteristics than typical cone units. Planar magnetics are at a disadvantage when compared to electrostatic type planar loudspeakers which exhibit better linearity. They are also at a disadvantage when compared to direct radiator cone loudspeakers with respect to efficiency.
The disadvantages of planar magnetics when compared to electrostatic type planar loudspeakers are the result of several factors. The electrostatic loudspeakers have the advantage of being able to drive a diaphragm uniformly over its entire area, which prevents a diaphragm breakup, as well as to drive it linearly by means of a constant charge operation.
An inherent problem with planar magnetic loudspeakers is driving the diaphragm uniformly across the entire area without becoming non-linear. See discussion in the patent to Torgeson, U.S. Pat. No. 4,468,530. The planar magnetic designs which have magnets on only one side of the diaphragm are inherently non-linear as indicated in the patent to Winey U.S. Pat. No. 3,674,946. The planar magnetic loudspeakers with like poles opposed on either side of the diaphragm are efficient, but have been demonstrated to have a linear magnetic field only within a narrow region between the magnets as indicated in the Torgeson patent. If conductors are placed across the entire area of the diaphragm, non-linearity results. The only known prior art type of planar magnetic loudspeaker that can be linear and have uniform drive area is one where the magnets are located on both sides of the diaphragm with alternate poles and with the magnets staggered so that flux lines run diagonally across the diaphragm. This approach in order to be linear must have conductors substantially covering the entire diaphragm as indicated in the patent to McKay U.S. Pat. No. 3,939,312. Because of the diagonal flux lines, the transfer efficiency is lower than that of other planar magnetics, electrostatics, and cone type loudspeakers.
2. Description of the Prior Art
The Patent to McKay U.S. Pat. No. 3,939,312 discloses a transducer having sheets of magnets in which the poles are alternately spaced so that the flux lines are disposed in a zig-zag fashion across the space between the two lines of magnets. Conductors are positioned in the space to intercept the magnetic flux that is present in the zones between the magnets. The plates which carry the magnets are pierced with a pattern of holes to permit passage of the sound vibrations from the diaphram on which the conductors are mounted.
The Patent to High U.S. Pat. No. 1,815,564 also discloses the use of magnetic poles alternately arranged providing a flux field between which the diaphram carrying conductors is positioned.
The Patent to Winey U.S. Pat. No. 3,674,946 discloses the use of a plurality of vibratable diaphrams formed to vibrate independently of each other and which are electrically isolated in an electromagnetic transducer having magnetic material located on one side of the diaphram.
Other Patents to Winey U.S. Pat. Nos. 4,471,172; 4,471,173; and 4,480,155 disclose elongated magnet strips carried on a sheet of magnetic material or connected by narrow bridges and mounted in pairs so that the flux field passes between adjacent magnets and through the conductors on a diaphram in a space between oppositely positioned magnets. Various arrangements of magnets and conductors are disclosed. The conductors may be formed in various ways, such as with round wire, foil, or metal film deposited on the film and etched away. A multiplicity of parallel conductors in a band is also disclosed.
The Patent to Whelan U.S. Pat. No. 4,156,801 discloses a diaphram having conductors thereon in a space between opposed pairs of magnets of like polarity and in which undriven areas of the diaphram are baffled to minimize the effects of vibrations.
The Patent to Torgeson U.S. Pat. No. 4,468,530 discloses a diaphram having conductors thereon in a space between opposed pairs of magnets of like polarity, the diaphram being divided into sections by means of rubber separator strips, and the arrangement of magnets within the various sections differing in order to provide zones of different frequency.